The TCP/IP protocol suite is made of five layers: physical, data link, network,
transport,andapplication.Thefirstfourlayersprovidephysicalstandards,networkinterfaces,
Inter-networking, and transport functions that correspond to the first four layers of the OSI
model.The three topmost layers in the OSI model, however, are represented in TCP/IP by a single
layercalledthe applicationlayer
PhysicalandDataLinkLayers:
Atthephysicalanddatalinklayers,TCP/IPdoesnotdefineany specificprotocol.
It supports all the standard and proprietary protocols. A network in a TCP/IP internetwork
can be alocal-areanetwork or awide-area network.
NetworkLayer:
At the network layer (or, more accurately, the internetwork layer), TCP/IP supports
theInternetworking Protocol. IP, in turn, uses four supporting protocols: ARP, RARP, ICMP,
andIGMP.
TransportLayer:
TraditionallythetransportlayerwasrepresentedinTCP/IPbytwoprotocols:
TCPand UDP. IP is a host-to-host protocol, meaning that it can deliver a packet from one
physical deviceto another. UDP and TCP are transport level protocols responsible for
delivery of a messagefrom a process (running program) to another process. A new transport
layer protocol, SCTP, hasbeendevised to meet the needs of somenewer applications.
a. UserDatagramProtocol
The User Datagram Protocol (UDP) is the simpler of the two standard TCP/IP transportprotocols. It
is a process-to-process protocol that adds only port addresses, checksum
errorcontrol,andlengthinformation to thedatafromthe upper layer.
b. TransmissionControlProtocol
TheTransmissionControlProtocol(TCP)providesfulltransport-layerservicesto applications.
TCP is a reliable stream transport protocol. The term stream, in this context, meansconnection-
oriented: A connection must be established between both ends of a
transmissionbeforeeithercantransmit data.
c. StreamControlTransmissionProtocol
The Stream Control Transmission Protocol (SCTP) provides support for newer applicationssuch
as voice over the Internet. It is a transport layer protocol that combines the best features
ofUDPand TCP.